The present invention relates to a submarine casing for a submarine optical cable in a submarine communications system.
A submarine optical-fibre communications system comprises high-capacity long-haul transmission links between land masses. The main trunk of these links connects two remote land masses, each land mass having an end station where the submarine cable is terminated. The main trunk may also include a branching unit for connecting the main trunk to another land mass via a spur to provide an additional transmission link. An example of a branching unit is shown in FIGS. 1 and 2, As shown, the branching unit 1 has three flexible coupling zones 2, each housing an extremity box 3 which provides a terminus for a submarine cable 4 connecting the submarine cable 4 to optical and/or electrical devices inside the branching unit 1. FIG. 2 shows the prior art protective features for the extremity box 3 and submarine cable 4. The cable may carry electrical as well as optical signals; it may include an electrical power lead. The extremity box 3 and the submarine cable 4 are covered in a polyethylene casing 5 to provide high voltage insulation. The submarine cable 4 and casing 5 have strength member 6 (stranded longitudinal wires or threads) and are enclosed in armour, to prevent physical damage. The armour 11 is helical winding(s) of metal wire or strip. Its outer surface may be covered in tar 7.
In normal use, devices in the branching unit are powered through a power lead in at least one cable 4, from its associated end station to a sea-earth (not shown) at the branching unit 1. The sea-earth typically comprises a length or a block of metal exposed to the sea and coupled to electrical relays in the branching unit 1. Current flows through the sea-earth causing electrolysis of the surrounding sea water leading to the generation of large quantities of hydrogen gas, typically in excess of 15 liters per day. The partial pressure of hydrogen around the branching unit 1 is normally minimised due to tidal flow. However, it is not uncommon for the branching unit 1 to become buried making it harder for hydrogen to escape and consequently hydrogen builds up around the branching unit 1. In this environment, hydrogen is able to diffuse into the submarine casing either directly into the extremity box 3 or along the polymer casing 5 of the submarine cable 4 and then into the extremity box 3.
Ingress of hydrogen directly into the extremity box 3 occurs through joints and fixings in the extremity box 3. Although the extremity box 3 is covered with a layer of polyethylene to provide high voltage insulation, this provides no effective barrier to the ingress of gas since polyethylene is permeable to hydrogen. In an optical fibre cable 4, glass fibre(s) run though a protective copper casing enclosed in the polyethylene casing 5 and which acts as the power lead to the branching unit 1. The hydrogen can diffuse through the polyethylene casing 5 but not through the copper casing. However, when the hydrogen diffuses through the polyethylene casing 5 it passes along the outside of the copper casing until it reaches the cable terminus at the extremity box 3, and once inside the extremity box 3, hydrogen can pass inside the copper casing where it diffuses into the interstices of the glass fibres leading to optical loss. Repairing the optical fibres in a submarine cable 4 underwater is a major undertaking and results in a loss of traffic. Clearly this situation is highly undesirable and needs to be avoided.
One possible solution is to locate the sea-earth at a sufficient distance from the submarine casing so that hydrogen cannot diffuse into the casing, usually at least 5 meters or so from the branching unit 1. This form of sea-earth is bulky and awkward to handle and makes it extremely difficult for the cable ship to lay the submarine cable 4. It is also difficult to guarantee the correct positioning of the sea-earth on the sea bed.
The branching unit 1 is not the only part of this submarine system which has an integral or associated sea-earth. Other connectors (e.g. repeaters) may be powered and have a sea-earth and present the same problem of major hydrogen generation. However, other parts of the submarine cable 4 may unintentionally form an electrical contact with sea water to create a short circuit, for example where two lengths of submarine cable 4 are joined. This will also lead to the generation of hydrogen which even though it generated in much lower quantities will still if there is corrosion or breakage diffuse into the optical fibres and cause high optical loss.
According to the present invention, a submarine casing for a submarine optical cable comprises a bonded metal sheath to reduce the ingress of hydrogen into the casing when submerged.
In JP-A-53111495 it has been proposed to adhesively wrap an aluminium tape helically around a join in an aluminium conductor of an underwater coaxial cable so as to prevent ingress of water at the joint and consequent corrosion.
In the present invention, however, the metal sheath provides an effective barrier against the ingress of hydrogen. The sheath may be a wrapping of metal. The metal is impermeable to hydrogen. The wrapping may be a spiral to increase the leak path length of the bonding layer to further reduce the ingress of hydrogen by diffusion through it.
Alternatively, the metal sheath may comprise a sleeve. Preferably, the sleeve comprises a sleeve of a heat-shrinkable material having a circumferentially concertina-able metal inner liner.
Preferably, the metal sheath comprises the combination of a number of metal sheets wrapped at least once around the casing and a number of fitted sleeves. Most preferably, the end of each fitted sleeve overlaps a portion of the metal sheet; the sheath then completely encloses the inside of the casing in metal.
Preferably, the metal is bonded using an adhesive. Preferably, the adhesive is a hot melt adhesive, an epoxy adhesive or an acrylic adhesive.
The present invention also provides a method of manufacturing a submarine casing comprising the step of bonding a metal sheath to the casing.
The present invention further provides a branching unit or cable connector comprising a submarine casing according to the present invention and with a sea-earth associated with the unit or connector.